1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser (VCSEL) emitting a laser beam from its top face and, more particularly, to a VCSEL suitably applicable to a use necessitating a beam profile of a perfect circle shape.
2. Description of the Related Art
A VCSEL emits light in a direction orthogonal to a substrate different from an end-face emitting type of related art. Since a number of elements may be arranged on the same substrate in a two-dimensional array shape, in recent years, the VCSEL is noted as a light source for a digital copy machine or a printer machine.
In the past, in a laser diode of this kind, a pair of multilayer reflecting mirrors is formed on a substrate, and an active layer as a light emitting region is provided between the pair of multilayer reflecting mirrors. For one of the multilayer reflecting mirrors, to increase the current injection efficiency to the active layer and decrease a threshold current, a current confinement layer having a structure in which a current injection region is narrowed is provided. A lower electrode is provided on the under face side, and an upper electrode is provided on the top face side. A light emission port for emitting a laser beam is provided in the upper electrode. In the laser diode, current is confined by the current confinement layer, and the confined current is injected to the active layer where light is generated. The light is repeatedly reflected by the pair of multilayer reflecting mirrors, and a resultant light is emitted as a laser beam from the light emission port in the upper electrode.
In the VCSEL, symmetric property in a structure in a plane (stack-layer plane) perpendicular to a direction of emitting a laser beam (stack-layer direction) is excellent, so that a beam shape close to a perfect circle may be obtained. It is known that fundamental transverse mode oscillation mainly occurs in a center region in the laser beam emission region and, on the other hand, mainly high-order transverse mode oscillation occurs in an outer edge region. Consequently, when the current injection region in the current confinement layer is made too wide or when the injection current amount is set too large for higher output, a laser beam in the high-order transverse mode is also output with high output power, and there is a disadvantage that a beam shape close to a perfect circle may not be obtained.
To solve such a disadvantage, a number of methods for creating a gain difference between a fundamental transverse mode and a high-order transverse mode have been proposed. For example, Japanese Unexamined Patent Application Publication Nos. 2001-210908 and 2004-288674 disclose a method of increasing a gain in the fundamental transverse mode by adjusting each of a current confinement diameter and the diameter of the light emission port in the upper electrode. For example, another literature discloses a method of forming a stack-layer structure of a semiconductor or an insulator in a center region in a laser beam emission region to increase reflectance, thereby increasing the gain in the fundamental transverse mode.